Textile azo dye decolorization and detoxification using bacteria isolated from textile effluents

Azo dyes, which are highly toxic, carcinogenic, and mutagenic for living organisms, are used as coloring chemicals in textile industries. Physicochemical methods used for removing azo dyes are expensive, can generate secondary waste, and are not very efficient. In this study, we used a biological approach to reduce the toxicity of three azo dyes, i.e., congo red, methyl orange, and reactive red 198, from textile effluents. Six dye-decolorizing bacteria were screened from waste water obtained from the textile industry (in the dyeing process) at a concentration 100 mg/l for each azo dye. Using a 16S rRNA approach, the bacteria were identified and assigned as Enterococcus faecalis VTK04, Staphylococcus aureus VTK013, Pseudomonas aeruginosa VTK018, Proteus mirabilis VTK024, Bacillus cereus VTK035, and Enterococcus faecium VTK054. These bacteria were tested for their ability to produce biofilms on the abiotic surface. The adherence assay showed that VTK013, VTK054, and VTK024 had a potential to form stable biofilms on abiotic surfaces (OD570 = 1.37). Moreover, the dye decolorization potential was spectrophotometrically measured after seven days with and without a carbon substrate. The results demonstrated that the absence of the carbon source had a negative impact on decolorization, whereas a carbon-supplemented medium showed a considerable increase in the decolorization of congo red (80% and 96% by VTK013 and VTK018, respectively), methyl orange (100% and 75% by VTK054 and VTK035, respectively), and reactive red 198 (90% and 86% by VTK04 and VTK024, respectively). The phytotoxicity study of the treated dye solutions showed lesser toxicity compared to the untreated dye solution. These results support the possibility of using bacterial isolates for the biodegradation of azo dye effluents.